$500 a year for hosting? Well, you have a relatively small forum, so what kind of service do you have from your host? This isn't a dedicated is it, if so, I would recommend getting an inexpensive but very reliable $20 host, such as Vervehosting (my host).
Otherwise, if you have regular old hosting, are you talking about almost $40/month you have to spend? $45 domain renewels??
I pay $15 annual domain renewels, as well $15/month (soon I
should upgrade to $20).
If you were to prune all those members to leave you 36, then justifying your hobby would be easy enough if you only have to pay $10 hosting and $15 annually for registration-- right?
Anyway, a forum usually is meant to be open, and I don't recommend closing it for only pay members. Actually, a paid-registration forum seems alittle odd actually, but I understand you need ways to pay for your hosting.
Hmm.. maybe
T-shirts?